Middle Tertiary Planktonic Foraminiferal Facies, San Joaquin Basin, California: ABSTRACT

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Biofacies analyses of the middle Tertiary marine units of the San Joaquin basin, California, show that the most continuous deep-water areas were near the southern end of the basin and close to the San Andreas fault system which bounds the basin on the west. Isopach maps demonstrate that the major sediment accumulation was generally in the more rapidly subsiding and deeper water areas of the basin, suggesting that mechanisms such as turbidity currents, sand flow, etc., must have been important in the deposition there. Planktonic foraminiferal facies are restricted to the southern part of the basin. Minor planktonic facies occurred in the Zemorrian and Saucesian Stages, perhaps 28 to about 20 million yr B.P. A major expansion of planktonic facies occurred during the Relizian Stage, about 20 to perhaps 17 million yr B.P. Only minor planktonic faunas are identified with the Luisian Stage, about 17 to perhaps 14 million yr B.P. A second major expansion of planktonic faunas occurred during the Mohnian Stage, perhaps 14 to about 11 million yr B.P. After the Mohnian, the deeper oceanic connection was attenuated rapidly and planktonic faunas disappeared in the latest Miocene. Abundance variations and distribution patterns of middle Tertiary planktonic foraminiferal facies of the San Joaquin basin indicate that (1) there was an important deep-water opening toward the west across the San Andreas fault system, (2) other possible connecting channels must have been of a much smaller magnitude, allowing no major influx of oceanic planktonic populations, (3) the main distribution pattern is counterclockwise indicating a similar counterclockwise current pattern around the southern boundary of the basin, (4) there were two major expansions End_Page 518------------------------------ of the current system and planktonic facies, and (5) the shallow-water facies west of the San Andreas fault at the south end of the San Joaquin Valley are anomalous. The facies anomaly across the San Andreas fault is consistent with the concept of major right lateral-slip movement since the middle Tertiary. End_of_Article - Last_Page 519------------